Re: [pvusers] Question on inverting DC to AC

["Scott Willing" <willing@mb.sympatico.ca>]

On 7 Sep 2002 at 20:46, Christopher Witmer wrote:

> Is the process of converting DC to AC simplified by keeping the
> voltage the same? For example, converting 120VDC to 120VAC. I ask
> because I'd like to be able to save money on an inverter for a system
> I'm contemplating. If I could eliminate most of the expense of an
> inverter I'd be thrilled . . .

Christopher,

1. The peak voltage of a 120VAC sine waveform is around 170V; if you 
want to a step-up stage, that's the voltage you need.

2. You need a mechanism to regulate your output voltage, and you also 
need transformer isolation between the DC source and the AC output 
(whether this is a low-frequency transformer in the output stage, or 
a high-frequency one in the DC/DC converter stage). So, while having 
a high-voltage DC source will mitigate certain design challenges, the 
fundamental topology ends up being the same.

3. High DC voltage is potentially dangerous stuff. Obviously, any 
high voltage is potentially dangerous, but whereas AC gives you a 
"buzz," DC clamps the muscles, which can make it tricky to let go. 
One of the most dangerous operations in a home power system is 
battery maintenance. It's bad enough that you can vaporize a wrench 
at relatively low voltages without having lethal DC to worry about.

4. If you put together a ~170VDC battery bank, you aren't going to 
find off-the-shelf charge controllers for it, so you're going to be 
building your own charger(s) as well.

5. Speaking as a technologist who (admittedly long ago) worked on 
inverter and battery charger designs for a UPS company, I advise you 
not to underestimate the challenges of building an inverter. Even 
with my background, I wouldn't dream of doing it today - not for a 
one-off, and not considering the price of modified square wave 
inverters. Getting by with one of these (rather than a "true" sine 
model) would be a lot better way to save money IMHO.

Hope I'm close to the target here.

-=s

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